The long-range objective is to understand enough about the factors that control mammalian cell growth to be able to apply this knowledge to the control of malignant growth. As a result of recent work, a broad picture is now available of the control of growth of mammalian cells. Application of this knowledge requires additional information and experimentation. Detailed information is needed of the specific factors that control the growth of the epithelial cells that are most important in the origin of tumors, in order to design rational procedures to restrict tumor growth. Among other things, it is important to know whether these epithelial cells produce inhibitors of their own growth, such as the growth inhibitors that are produced by kidney epithelial cells. The kidney epithelial cell growth inhibitor is active on certain lung and mammary cells in culture. With CCL 64 mink lung cells, 60% inhibition of [3H]thymidine incorporation is observed at a 0.1-nanogram concentration of the growth inhibitor. Injection of the growth inhibitor in vivo into human mammary carcinomas growing in nude mice inhibits [3H]thymidine incorporation into the tumors.